Cold Cathode Fluorescent Lamp (CCFL) is used to provide backlight to display systems in laptop computers. While most voltages in laptop computers are relatively small in magnitude, the voltage that powers to a CCFL is typically in the order of thousands volts in magnitude. Today, most laptop computers are typically driven by a full bridge power stage that drives a magnetic step-up transformer that provides the required high voltage to the CCFL loads. In this manner, a supply voltage for a laptop computer having a typical voltage of 7 to 22 volts can efficiently regulate a 600 VRMS voltage to the CCFL. However, the high voltage applied to the CCFL may cause dangerous electrocution to users. For this reason, manufacturers are required to implement redundant physical and electrical safety systems to protect consumers from electrocution by their laptop computers.
Additionally, most laptop computers are only commercially viable if they pass standard tests known as the Underwriters Laboratory (U.L.) Standards 1950. In U.L.'s Standards 1950, there are tests designed to determine if products meet health and safety standards. One common test for electrical devices is whether the product would drive too much current through a human body model load. Another common test is whether the product operates safely (or shuts down) when any two physically accessible components are short-circuited—a component short or a short of a component to ground. When such short-circuit conditions happen, U.L.'s Standards require that the laptop to either shut down immediately or limit the operating current to a negligible amount. Thus, it may be desirable to provide a robust fault detection circuit connected to electrical devices, e.g., CCFL loads in laptop computers, which meet the U.L.'s 1950 Standards.
In response, there are many prior-art attempts to pass the U.L.'s 1950 Standards. One of these prior art is shown in FIG. 1 which is an overload sensing circuit 100. Overload sensing circuit 100 includes a transformer 110 electrically coupled to drive a load 120. Load 120 provides a reference voltage at a node A. A first comparator 130 and a second comparator 140 are electrically coupled to load 120 at node A to receive the reference voltage. The other input terminal of first comparator 130 is a band gap voltage; while the other input terminal of second comparator 140 is coupled to receive a band gap voltage divided by 10. When overload sensing circuit 100 operates properly, the output of first comparator 130 and second comparator 140 are typically square waves, which may be difficult to detect improper operation. Furthermore, during fault condition, transformer 110 may saturate. As a result, transformer 110 delivers less current than a specified amount of current to human body model load. When this happens, load 120 does not generate sufficient load current. Thus, comparators 130 cannot accurately report such fault condition.
Referring now to FIG. 2, another prior art current overload sensing circuit 200 is illustrated. In overload sensing circuit 200, instead of using a test of voltage at the output of transformer 110, voltage at the input side (primary winding) may be measured. A first buffer 220 and a second buffer 230 represent the power bridge which drives a transformer 210. A capacitor 240 is coupled in series between second buffer 230 and a terminal B of the primary winding of transformer 210. In addition, a Zener diode 250, a diode 250, a resistor 270, and a common-base transistor 280 are also coupled in series to terminal B. The other side of common-base transistor is a terminal 290. Terminal 290 can be coupled to a comparator (not shown) to determine if the conditions at node B of transformer 210 are reasonable. However, each of the components such as Zener diode 250, diode 260, and common-based transistor 280 are discrete components on a circuit board, which may be shorted or opened in a U.L. test or by a component failure and results in a failure of overload sensing circuit 200. Thus, overload sensing circuit 200 illustrated in FIG. 2 introduces more complexity without providing more robust tests that pass the U.L.'s 1950 Standards.
What may then be useful is a testing scheme which is robust—relatively easy to measure and relatively unlikely to result in failures due the shorting or contact tests within the U.L.'s 1950 Standards.